Radiation sensitive silver halide emulsions containing one or a combination of chloride, bromide and iodide ions have been long recognized to be useful in photography. Each halide ion selection is known to impart particular photographic advantages. By a wide margin the most commonly employed photographic emulsions are silver bromide and bromoiodide emulsions. Although known and used for many years for selected photographic applications, the more rapid developability and the ecological advantages of high chloride emulsions have provided an impetus for employing these emulsions over a broader range of photographic applications. As employed herein the term "high chloride emulsion" refers to a silver halide emulsion containing at least 50 mole percent chloride, based on total silver. The most ecologically attractive high chloride emulsions are those that contain no iodide ion or very low levels of iodide ion.
During the 1980's a marked advance took place in silver halide photography based on the discovery that a wide range of photographic advantages, such as improved speed-granularity relationships, increased covering power both on an absolute basis and as a function of binder hardening, more rapid developability, increased thermal stability, increased separation of native and spectral sensitization imparted imaging speeds, and improved image sharpness in both mono- and multi-emulsion layer formats, can be realized by increasing the proportions of selected tabular grain populations in photographic emulsions.
Although varied definitions have been adopted in defining tabular grain emulsions, there is a general consensus that the functionally significant distinguishing feature of tabular grains lies in the large disparity between tabular grain equivalent circular diameter (ECD, the diameter of a circle having an area equal to the projected area of the tabular grain) and tabular grain thickness (t, the dimension of the tabular grain normal to its opposed parallel major faces). Average tabular grain aspect ratio (ECD/t) and tabularity (ECD/t.sup.2, where ECD and t are each measured in .mu.m) are art accepted quantifiers of this disparity. To distinguish tabular grain emulsions from those that contain only incidental tabular grain inclusions it is also the recognized practice of the art to require that a significant percentage (e.g., greater than 30 percent and more typically greater than 50 percent) of total grain projected area be accounted for by tabular grains.
An emulsion is generally understood to be a "high aspect ratio tabular grain emulsion" when tabular grains having a thickness of less than 0.3 .mu.m have an average aspect ratio of greater than 8 and account for greater than 50 percent of total grain projected area. The difficulty in achieving high average aspect ratios in high chloride tabular grain emulsions has often led to accepting average aspect ratios of greater than 5 as the best available approximations of high average aspect ratios. The term "thin tabular grain" is generally understood to be a tabular grain having a thickness of less than 0.2 .mu.m. The term "ultrathin tabular grain" is generally understood to be a tabular grain having a thickness of 0.06 .mu.m or less. High chloride thin tabular grain emulsions have been difficult to prepare and ultrathin high chloride tabular grain emulsions have been completely unknown.
In almost every instance tabular grain emulsions satisfying grain thickness (t), average aspect ratio (ECD/t), average tabularity (ECD/t.sup.2) and projected area aims have been formed by introducing two or more parallel twin planes into octahedral grains during their preparation. Regular octahedral grains are bounded by {111} crystal faces. The predominant feature of tabular grains formed by twinning are opposed parallel {111} major crystal faces. The major crystal faces have a three fold symmetry, typically appearing triangular or hexagonal.